The present invention relates to a ghost cancelling method and apparatus using canonical signed digit codes, and more particularly to a ghost cancelling method and apparatus capable of cancelling ghosts which exist in a television broadcast signal on the basis of filter coefficients converted into canonical signed digit codes.
A ghost phenomenon or a multipath distortion is due to a result of a multipath channel which occurs due to the reflectance of radio waves by mountains, buildings or airplanes. The ghosts are displayed on a television receiver set as different patterns of video signals superimposed on an image of an original signal and having delay and attenuation characteristics with respect to the original image signal. The ghost also distorts a frequency characteristic of the television signal. There have been various approaches for reducing ghosts since the ghost phenomenon significantly lowers the picture quality of an image displayed on the television receiver set. These approaches are disclosed in the following references: [1] "A Tutorial On Ghost Cancelling In Television Systems" (IEEE Transactions On Consumer Electronics, Vol. CE-25, pp. 9-44, February 1979, by W. Ciciora, G. Sgrignoli and W. Thomas); [2] "Training Signal And Recursive Design For Multi-path Channel Characterization For TV Broadcasting" (IEEE Transactions On Consumer Electronics, Vol. 36, No. 4, pp. 794-806, November 1990, by J. D. Wang, T. H. S. Chao and B. R. Saltberg); [3] "Development Of A Ghost Cancel Technology For TV Broadcasting" (1990 NAB Engineering Conference Proceedings, pp. 229-238, 1990, by Shigeo Matsuura et al); and [4] "Ghost Cancelling: A New Standard For NTSC Broadcast Television" (IEEE Transactions On Broadcasting, Vol. 38, No. 4, pp. 224-228, December 1992, by Lynn. D. Claudy and Stephen Herman).
The above reference [3] proposed a system for automatically cancelling ghosts using a waveform equalization technology. According to reference [3], a transmitter inserts a ghost cancelling reference (GCR) signal in a vertical blanking period of a video signal and transmits the result, so that a receiver can effectively cancel the ghosts. The reference [4] a standard GCR signal which is used in the United States in order to cancel ghosts. A reference [5] relating to the standard Korean GCR signal is entitled "Reference Signals For Ghost Cancelling In Television" (ITU-R Document 11C/46-E).
Most ghost cancelers use a number of transversal filters to reduce ghosts. A general transversal filter for cancelling ghosts should have a number of taps and multipliers for filtering a video signal of one horizontal scan line period. This requires a large-sized chip and raises a problem of large power consumption. Thus, this arrangement is disadvantageous in that it is difficult to implement in a single chip a transversal filter for cancelling ghosts.
U.S. Pat. No. 4,953,026 to Reiichi Kobayashi et al. entitled "Video Ghost Signal Cancelling Circuit Having Variable Delay Lines" discloses a method for reducing the number of taps of filters to implement a transversal filter into a single chip. Actually, as in the above U.S. Pat. No. 4,953,026, variable delays are used to form a filter having a low number of taps, to thereby reduce the hardware complexity of a filter. However, since multipliers constituting such a filter still occupy a large space and have large power requirements, it is desirable to simply implement such multipliers.